The recovery of ethylene from crude light hydrocarbon gas mixtures is an economically important but highly energy intensive process. Cryogenic separation methods are commonly used which require large amounts of refrigeration at low temperatures, and the continuing development of methods to reduce refrigeration power is important for olefin recovery in the petrochemical industry.
Ethylene is recovered from light gas mixtures such as cracked gas from hydrocarbon crackers which contain various concentrations of hydrogen, methane, ethane, ethylene, propane, propylene, and minor amounts of higher hydrocarbons, nitrogen, and other trace components. Refrigeration for condensing and fractionating such mixtures is commonly provided at successively lower temperature levels by ambient cooling water, closed cycle propane/propylene and ethane/ethylene systems, and work expansion or Joule-Thomson expansion of pressurized light gases produced in the separation process. Numerous designs have been developed over the years using these types of refrigeration as characterized in representative U.S. Pat. Nos. 3,675,435, 4,002,042, 4,163,652, 4,629,484, 4,900,347, and 5,035,732.
An improvement to the cryogenic separation methods described above is disclosed in U.S. Pat. No. 4,002,042 wherein the final cooling and condensing of the feed gas between about -75.degree. F. and -175.degree. F. is performed in a dephlegmator type heat exchanger. This provides a much higher degree of prefractionation as the ethylene-containing liquids are condensed from the cold feed gas, since the dephlegmator can provide 5 to 15 or more stages of separation compared to the single stage of separation provided by a partial condenser type of heat exchanger. As a result, significantly less methane is condensed from the feed gas and sent to the demethanizer column and refrigeration energy requirements for both feed cooling and demethanizer column refluxing are reduced. This improved process combines a dephlegmator with a demethanizer column to achieve energy savings in both the cryogenic separation and cold fractionation sections of the ethylene plant.
Further improvements to the cryogenic separation and cold fractionation sections of the conventional process are described in U.S. Pat. 4,900,347. In these improvements, all feed gas cooling for ethylene recovery below about -30.degree. F. is carried out in a series of at least two dephlegmators, for example, a warm dephlegmator and a cold dephlegmator, and the demethanizer column is split into a first (warm) demethanizer column and a second (cold) demethanizer column, both operating at high pressure. Some feed cooling above -30.degree. F. may also be done in a dephlegmator. The warm dephlegmator condenses and prefractionates essentially all of the propylene and heavier hydrocarbons remaining in the -30.degree. F. feed gas along with most of the ethane and this liquid is sent to the warm demethanizer column. Reflux for the warm demethanizer column is typically provided by condensing a portion of the overhead vapor using propylene or propane refrigeration at -40.degree. F. or above. The bottom liquid from the warm demethanizer column is sent to the de-ethanizer column where the C.sub.3 and heavier hydrocarbons (C.sub.3.sup.+) are recovered as a bottom product. The C.sub.2 hydrocarbon overhead from the de-ethanizer column is sent to the ethylene/ethane splitter column. The cold dephlegmator condenses and prefractionates the remaining ethylene and ethane in the cold feed gas and this liquid is sent to the cold demethanizer column. Reflux for the cold demethanizer column is typically provided by condensing a portion of the overhead vapor using ethylene refrigeration at about -150.degree. F. The ethylene-rich bottom liquid from the cold demethanizer column contains essentially no propylene or propane and is sent directly to the ethylene/ethane splitter column as a second feed, thus bypassing the de-ethanizer column.
U.S. Pat. No. 5,035,732 describes a variation of the process described above wherein the second (cold) demethanizer column is operated at low pressure conditions, 175 psia or less. Reflux for the low pressure cold demethanizer column is provided by condensing a portion of the cold demethanizer column overhead vapor or the cold dephlegmator overhead vapor, using expander and/or other process stream refrigeration below -150.degree. F.
The improved processes of U.S. Pat. Nos. 4,900,347 and 5,035,732 combine multiple dephlegmators with a multi-zone demethanizer column system to achieve energy savings in the cryogenic separation section of the ethylene plant, and to achieve both capital and energy savings in the cold fractionation section of the ethylene plant. Compared to the conventional process:
1) the dephlegmators require less refrigeration energy than conventional partial condenser type heat exchangers because significantly less methane is condensed; PA1 2) the multi-zone demethanizer column system is cheaper than the conventional single-column demethanizer system because the warm column utilizes less expensive materials and the cold column, which uses more expensive materials, is smaller than the conventional single-column (cold) demethanizer; PA1 3) the multi-zone demethanizer column system requires less refrigeration energy for refluxing because less methane is condensed and sent to the columns and also because the warm column utilizes warmer, low energy intensive refrigeration and the cold column uses less cold, high energy intensive refrigeration than the conventional single-column (cold) demethanizer; PA1 4) the de-ethanizer column is smaller and requires less separation energy due to the smaller quantity of liquid which must be processed in the column; and PA1 5) the ethylene/ethane splitter column is smaller and requires less separation energy due to the preseparation provided by the two feed streams to the column.
The improved processes described in U.S. Pat. Nos. 4,002,042, 4,900,347 and 5,035,732 can be used to recover ethylene from feed gas produced by cracking of ethane, ethane/propane, or heavier hydrocarbons such as LPG, naphtha or gas oil.
Thus the use of a multi-zone demethanizer system is an efficient and preferred mode of operation for recovering ethylene from ethylene-containing feed gases. Further improvements to such a system are desirable, and such improvements are realized for ethylene-containing feed gases from ethane and ethane-propane cracking by the invention described in the following specification and defined by the appended claims.